Bituminously bound mixtures are produced in mixing systems. This is done by heating stone chippings in a rotary furnace and subsequently feeding them to a mixer. In this mixer, hot bitumen is additionally injected and mixed with the hot stone chippings. This mixture is then temporarily stored in hot silos or transported directly to the road construction site by means of trucks. The asphalt leaves the mixer at a high and very uniform temperature. The mixture cools in a non-uniform manner due to subsequent storage and particularly due to transportation. Typically, the asphalt still has a very high core temperature when delivered to the construction site, but the edge areas have cooled significantly. There is then no longer a mixture with a uniform temperature. A uniform temperature distribution in the mixture is one of the most important parameters for laying and compacting asphalt. Many material characteristics of the asphalt depend on this temperature. This is substantially associated with the viscosity of the bitumen, which viscosity changes with the temperature. A non-uniform temperature of the mixture is therefore a factor that has a negative influence on the quality of the road surface. It leads to density differences in the load-bearing capacity, as well as to flaws in the layer thickness and, as a result, to unevenness in the road surface.
These findings have been used and implemented in a feeder system that is intended to improve the homogeneity of the mixture temperature. To this end, one uses, e.g., conveying screws that are arranged in the material hopper such that they are transverse to the main conveyor flow. Such a system is for instance disclosed in WO 2007/117287 A1. Due to the conveying screws, colder mixture is continuously conveyed from the edge areas into the hotter main conveyor flow during the feeding process. This ongoing mixing leads to improved temperature homogeneity in the mixture. It is here important that a so-called tunnel effect is substantially avoided. This effect occurs in screws of a constant pitch and a constant outer diameter. As soon as the first winding of the screw has been filled with material, no further material can pass from the top into the winding, whereby the desired mixing effect fails to take place. The above-mentioned disclosure therefore describes conveying screws of a variable pitch and of a variable outer diameter, respectively, of the screw flank.
WO 2009/061278 A1 describes a conveyor device for a road construction vehicle. The conveyor device comprises a material hopper for accommodating paving mixture, the material hopper comprising two hopper halves with transverse conveying screws disposed therein. Owing to the transverse conveying screws the paving mixture is transported out of the hopper halves onto a longitudinal conveyor device. It is here possible to set the speed of the transverse conveying screws independently of the speed of the longitudinal conveyor device.
The applicant's EP 2 377 994 A1 discloses a similar method with a plurality of transverse conveying screws that can be operated independently of one another. A temperature measuring system is there used in addition.
EP 0 957 204 A1 discloses a road paver with transverse conveying devices that are arranged in hopper halves, various sections of the hopper halves being pivotable about the transverse conveying means such that residual material slides towards the transverse conveying means.
Further concepts for optimizing the temperature of the mixture are disclosed in DE 20 2008 010 719 U1 and in EP 1 213 390 A2, both issuing from the applicant. The first one provides an additional heater for the mixture, said heater exploiting the engine waste heat of the conveyor vehicle. The latter refers to a special configuration of a conveyor belt for a mixture that tends to adhere and/or to solidify.